The present invention relates to a semiconductor device including a layer of a gallium nitride (GaN) compound semiconductor generally represented by InxAlyGa1xe2x88x92xxe2x88x92yN (0xe2x89xa6X less than 1, 0xe2x89xa6Y less than 1, 0xe2x89xa6X+Y less than 1) (hereinafter, compound semiconductors of this group are collectively called as xe2x80x9cGaN compound semiconductorsxe2x80x9d) and a method for fabricating such a semiconductor device. More particularly, the present invention relates to a Schottky electrode formed in contact with a GaN compound semiconductor layer and a method for forming such a Schottky electrode.
GaN compound semiconductors such as GaN, AlGaN, InGaN, and InAlGaN are direct transition semiconductors having a band gap varying in the range from 1.95 eV to 6 eV. These semiconductors are therefore expected as promising materials for light emitting devices such as laser diodes. GaN is also expected as a promising material for a high-frequency power device since it possesses high dielectric breakdown electric field intensity, high thermal conductivity, and a high electron saturation rate. In particular, an AlGaN/GaN heterojunction structure has an electric field intensity as high as 1xc3x97105 V/cm and an electron velocity twice or more as high as that of GaAs. This structure is therefore expected to contribute to realization of high-frequency operation in combination with miniaturization of the device.
GaN compound semiconductors exhibit n-type characteristics when they are doped with an n-type dopant such as Si and Ge. Therefore, it has been attempted to apply the GaN compound semiconductors to field effect transistors (FETs). In general, a metal semiconductor field effect transistor (MES-FET) using a Schottky metal as a Schottky electrode has been studied. Schottky characteristics greatly influence the drain breakdown voltage and the current characteristics of a FET obtained when the gate voltage applied is positive. Conventionally, therefore, in consideration of the Schottky characteristics, metal such as palladium and platinum is generally used as the Schottky electrode for a GaN compound semiconductor.
However, although the metal such as palladium and platinum is good in barrier height and ideal factor n value as indicators of the Schottky characteristics, it is poor in adhesion to a GaN compound semiconductor that is to form a Schottky junction together with the metal. As a result, the electrode is disadvantageously peeled off or lifts during fabrication process. Therefore, for a high-frequency device where a fine gate having a gate length of a sub-half micron is indispensable, in particular, processing of such a Schottky metal will be further difficult.
An object of the present invention is providing a semiconductor device including a Schottky electrode excellent in adhesion to a GaN compound semiconductor layer. Another object of the present invention is providing a method for fabricating such a semiconductor device.
A semiconductor device of the present invention includes: a gallium nitride compound semiconductor layer; and a Schottky electrode formed on the gallium nitride compound semiconductor layer, wherein the Schottky electrode contains silicon.
In an embodiment, the weight content of the silicon in the Schottky electrode is in a range between more than 0% and 20% or less.
In another embodiment, the weight content of the silicon is in a range between 3% and 20%, inclusive.
In still another embodiment, the Schottky electrode has been heat-treated at a temperature in a range between 400xc2x0 C. and 600xc2x0 C.
Another semiconductor device of the present invention includes: a gallium nitride compound semiconductor layer; and a Schottky electrode formed on the gallium nitride compound semiconductor layer, wherein the Schottky electrode contains nickel.
In an embodiment, the weight content of the nickel in the Schottky electrode is in a range between more than 0% and 20% or less.
Still another semiconductor device of the present invention includes: a gallium nitride compound semiconductor layer; and a Schottky electrode formed on the gallium nitride compound semiconductor layer, wherein the Schottky electrode has a multilayer structure, and the bottom layer of the multilayer structure in contact with the gallium nitride compound semiconductor layer is made of silicon.
Still another semiconductor device of the present invention includes: a gallium nitride compound semiconductor layer; and a Schottky electrode formed on the gallium nitride compound semiconductor layer, wherein the Schottky electrode has a multilayer structure, and the bottom layer of the multilayer structure in contact with the gallium nitride compound semiconductor layer is made of nickel.
In an embodiment, the thickness of the bottom layer is in a range between more than 0 nm and 20 nm or less.
In another embodiment, the Schottky electrode contains palladium or platinum.
In still another embodiment, a metal having a resistivity lower than the Schottky electrode is formed in contact with the Schottky electrode.
A method for fabricating a semiconductor device of the present invention includes the steps of: preparing a gallium nitride compound semiconductor layer; forming a Schottky electrode containing silicon or nickel on the gallium nitride compound semiconductor layer; and after the step of forming a Schottky electrode, heat-treating the Schottky electrode at a temperature in a range between 400xc2x0 C. and 600xc2x0 C.
Another method for fabricating a semiconductor device of the present invention includes the steps of: preparing a gallium nitride compound semiconductor layer; forming a Schottky electrode having a multilayer structure the bottom layer of which is made of silicon or nickel; and after the step of forming a Schottky electrode, heat-treating the Schottky electrode at a temperature in a range between 400xc2x0 C. and 600xc2x0 C.
In an embodiment, the step of heat-treating the Schottky electrode is performed at a temperature in a range between 500xc2x0 C. and 600xc2x0 C.